Why do whole aspen groves turn color at once?

In the Rockies, aspen color doesn’t spread tree by tree. Whole patches of hillside go gold together, while the patch next to them stays green. The reason is underground.

A grove can be one plant

Quaking aspen spreads mainly by sending up new shoots from its roots. Over decades, one tree can become a whole grove of trunks that are genetically identical and share the same root system. Each grove like this is called a clone.

Same genes, same timing

Because every trunk in a clone has the same genes, they respond to shorter days and cooler nights in the same way, at the same time. That’s why one patch turns gold all at once. The clone next to it has its own genes, so it can turn a few days earlier or later, or even a different shade. From across a valley, you can often see the outline of each clone as a block of color with a sharp edge.

The same thing happens in spring: whole clones leaf out together, and neighboring clones can be a week apart.

Gold, orange, and the occasional red

Most aspen clones turn a clear gold. A few turn orange or reddish, and because the color is set by the clone’s genes, those same groves tend to turn orange or red year after year.

One of the largest living things

Pando, an aspen clone in central Utah, covers about 106 acres, with tens of thousands of trunks growing from one root system. It’s considered one of the largest and heaviest living organisms known.

Fire and new groves

Because the roots survive when the trunks don’t, aspen often comes back quickly after a fire or avalanche, sprouting thickly from the old root system. Many of the big aspen stands in the West got their start after old fires.

Where to see it

Colorado is the classic place, with big aspen displays at Kenosha Pass, around Crested Butte, and near Telluride. Northern New Mexico, Utah, and the North Woods have aspen too. See Colorado’s peak calendar for typical timing.